High quality AV requires tight synchronization for MPEG-2TS, and for high quality audio broadcast in stereo, to different locations. These delay requirements are typically of the order of tens of nanoseconds.
In an ad hoc network especially, but also in situations where a Direct Link Protocol is used, as in 802.11e, the clock may not be accurate enough to meet these requirements or may not be able to meet synchronization requirements because of the presence of hidden stations. Without such synchronization capability, a higher “presentation” delay may be encountered as a result of attempts to synchronize from higher layers alone.
In a wireless network, stations transmit data to other data stations using Carrier-Sense Multiple Access/Collision Avoidance (CSMA/CA) as a primary mechanism to access the medium. For access to the medium, the stations must abide by certain channel access rules which include not occupying the medium for a certain time. Furthermore, when a station senses that the medium busy, it should backoff for a predetermined amount of time before again accessing the channel, to minimize the probability of collision. This requires that the clocks in all stations be synchronized. In the case of the delivery of data to certain applications, such as MPEG-2TS, or audio synchronization of remote outputs using IEEE 1394, tight clock synchronization between the transmitter and receiver, on the order of tens of nanoseconds, is required.
IEEE 802.11 defines mechanisms to synchronize clocks. In the so-called infrastructure mode, an Access Point (AP) maintains a clock, and distributes its value periodically. In an ad hoc network mode, the clock value is managed in a distributed manner. A station that “wins” contention at a particular instant, called Target Beacon Transmission Time or TBTT, will announce its clock value. During the next TBTT, a different station may win contention and send its own clock value.
While the mechanism in IEEE 802.11 is effective for both infrastructure and ad hoc modes, it makes an implicit assumption that there are no hidden stations, e.g., all stations can listen to other stations' transmissions. This assumption may not be valid in a typical home network using a PHY layer having a limited range, such as ultra-wide band (UWB), or when Direct Link Protocol (DLP) is used. This means that if two stations cannot hear each other, and if one has transmitted its clock value, it is very likely that the other will also transmit its own clock value and the stations “hearing” both transmitted clock values may synchronize to different clocks at various times, preventing the synchronization throughout the network.
U.S. Pat. No. 6,594,273 B1, for Self-configuring radio network, to McGibney, granted Jul. 15, 2003, deals with a distributed synchronization for TDMA-based system. A special synchronization signal is transmitted at a specified time.
U.S. Patent Publication No. 2004/0008661 A1, for Method and apparatus for synchronization in a wireless network, of Myles et al., published Jan. 15, 2004, describes decoding information in a packet and copying data from a local free-running clock at a known reference point.
U.S. Patent Publication No. 2004/0005902 A1, for System and method for correcting the clock drift and maintaining the synchronization of low quality clocks in wireless networks, of Belcea, published Jan. 8, 2004, describes how synchronization may be achieved between a clock server and low-cost clock clients.
U.S. Patent Publication No. 2003/0151513 A1, for Self-organizing hierarchical wireless network for surveillance and control, of Herrmann et al., published Aug. 14, 2003, describes a self-organizing wireless network in which the ACK frames sent by some STAs may optionally contain time stamps.
U.S. Patent Publication No. US2003/0117991 A1, (WO2003055103) for Apparatus, and an associated method, for facilitating synchronization in a wireless mesh network, of Beyer et al., published Jun. 26, 2003, describes synchronization for a mesh network and ad hoc networks.
U.S. Patent Publication No. 2003/0012168 A1, for Low-latency multi-hop ad hoc wireless network, of Elson et al., published Jan. 16, 2003, describes synchronization for a mesh network.
U.S. Patent Publication No. 2002/0131398 A1 (WO2002073851) for Maintaining a global time reference among a group of networked devices, of Taylor, published Sep. 19, 2002, describes synchronization between the clocks of two different networks.
WO2003073304 for Personal profile sharing and management for short range wireless terminals, of Heinonen et al., published Sep. 4, 2003, describes a synchronization mechanism in ad hoc networks.
Other publications relevant to the disclosure herein include IEEE 802.11-1999 Std. 2003 Edition; Meyr et al., Synchronization in Digital Communications: Volume 1, John Wiley & Sons Inc., New York, 1990, and Ogata, Modern Control Engineering, Prentice Hall Inc., Englewood Cliffs, N.J., 1970.